Matt Brown in Gaza:

A ceasefire would allow civilians to resupply but they will only have a six-hour window.

Israel's warned those in the northern and southern edges of Gaza City, people who've been told they must evacuate their homes, that they need to be out within five hours.

Israel also warned that it will retaliate if Hamas breaks the ceasefire.

The mood's getting much more tense. We were at an airstrike today at the home of what one person told us was a Hamas official and a man started yelling at us 'God will take revenge on you', 'You're the enemy', 'This is happening because of you'. So a fair deal of xenophobic feeling really.

When we went to the site of another missile strike, it came in right across our heads and crashed into land just a couple of blocks away. When we got there, there were Hamas men on the scene. Like we were there within minutes, Hamas men on the scene stopping us from filming altogether. So a pretty obvious example I think of a probable military installation of some sort right there in the middle of the city.

UN special coordinator for the Middle East peace process Robert Serry had earlier told Israeli TV that he had asked the military for a "humanitarian pause" in its Gaza offensive and that, if it agreed, he would ask Gaza militants to follow suit.

"If it happens, I will also call on Hamas and other militant factions to reciprocate, stop firing rockets and respect the humanitarian pause," his spokeswoman quoted him as saying.

"It is in the interest of all the citizens of Gaza, in order to give them a chance to go to the hospital, get health care or get food."

Militant group Hamas said it was considering the request.

"Hamas has been informed about the UN proposal for a truce of several hours tomorrow," Hamas spokesman Sami Abu Zuhri said.

"The proposal is being studied and when a decision has been taken it will be announced officially in a statement."

Four boys killed by Israeli fire on Gaza beach

Israel's attacks on Gaza continued ahead of the ceasefire, with several more Palestinians killed, while Hamas militants continued to fire rockets at Israeli cities.

In one incident, four boys aged between eight and 11 were killed on a Gaza beach when they were hit by Israeli fire in full view of several foreign journalists.

Israel's military said the deaths appeared to be the "tragic outcome" of an Israeli strike targeting Hamas militants.

"Based on preliminary results the target of this strike was Hamas terrorist operatives," it said in a statement.

"The reported civilian casualties from this strike are a tragic outcome."

Israel said it would investigate the incident.

The death toll in the conflict has now risen to more than 200 people, all but one of them Palestinian.

US working hard for Gaza truce, says Kerry

The United States was doing "everything in our power" to end the bloodshed in the Gaza Strip, secretary of state John Kerry said.

"Our concern is to have a legitimate ceasefire and see if we can find a way to stop the conflict and killing so we can get to the real issues that are underlying it," Mr Kerry said.

He added that he had been speaking to Israeli prime minister Benjamin Netanyahu and Arab officials.

Egypt's efforts to broker an end to the violence collapsed on Tuesday when Hamas, which rules Gaza and has fired more than 1,200 rockets into Israel, rejected the proposal on the grounds that it was not included in talks.

The US state department said Mr Kerry in the past day had spoken to the foreign ministers of Egypt, Qatar and the United Arab Emirates - which are seen as holding influence with Hamas - and to Arab League secretary-general Nabil al-Arabi.